add_filter("gform_after_submission", "nifty_add_post_attachements", 10, 3);
function nifty_add_post_attachements($entry) {
//you'll need this later, TRUST ME.
if(!function_exists('wp_generate_attachment_metadata')){require_once(ABSPATH . 'wp-admin/includes/image.php');}

//do we even have a file?
if (isset($_FILES['input_4'])) {
$file_url = $entry['4']; //great but what is its url?
$upload_dir = wp_upload_dir(); //where do you want to put it?
$file_data = file_get_contents($file_url); //show me what you're made of
$filename = basename($file_url); //so cute but what's its name?
if(wp_mkdir_p($upload_dir['path'])) //can we put it there?
$file = $upload_dir['path'] . '/' . $filename; //yes great
else //or no, okay fine let's try somewhere else
$file = $upload_dir['basedir'] . '/' . $filename; //get the whole location
file_put_contents($file, $file_data); // tada home at last

$wp_filetype = wp_check_filetype($filename, array('pdf' => 'application/pdf','pdf' => 'application/x-pdf') ); //is it the right type of of file?
$attachment = array( //set up the attachment
'post_mime_type' => $wp_filetype['type'],
'post_title' => sanitize_file_name($filename),
'post_content' => '',
'post_status' => 'inherit'
);

$attach_id = wp_insert_attachment( $attachment, $file, $entry['post_id'] ); //insert attachment
$attach_data = wp_generate_attachment_metadata( $attach_id, $file ); //asign the meta
wp_update_attachment_metadata( $attach_id, $attach_data ); //update the post

//remove the entry
//(to keep the entry comment or delete these line)
// IMPORTANT See:  https://pastebin.com/quvsvGHJ for the function
if(!function_exists('sedc_gform_remove_entries')){ die('next time maybe you should read the comments');}
else{ nifty_gform_remove_entries($entry, $form );}
}
}